Impassive hitman Jef Costello lives in a spartan single-room Paris apartment and keeps a small bird in a cage as a pet. His work follows a cold, methodical pattern: he builds airtight alibis, even ones provided by his lover, Jane Lagrange.
After carrying out a contract killing on Martey, a nightclub owner, Jef is clearly seen leaving the scene by the club's piano player, Valérie, and glimpsed by several other witnesses. The police bring Jef and other suspects in for a lineup, but the witnesses fail to positively identify him.
Jef is released, but the commissaire suspects him and has him followed. Jef loses the tail and goes to collect his fee for the hit. Instead of paying him, the man he meets tries to kill him, shooting him in the arm. Jef realizes his unknown employers now view him as a liability because he is suspected in Martey's murder. After treating his wound, Jef returns to the nightclub. While he is out, two police officers bug his room, agitating the bird in its cage.
After the nightclub closes, Jef has Valérie take him to her home, reasoning that she did not identify him as the killer because his employer had told her not to. He asks her who hired him, and she tells him to call her in two hours. Meanwhile, the police search Jane's apartment and offer to leave her alone if she recants her alibi for Jef. She flatly rejects the offer.
Back at his apartment, Jef notices some loose feathers scattered around his bird's cage and suspects it was agitated by an intrusion. He finds the police bug and deactivates it, and then goes to a public phone to call Valérie, but she does not answer. When he returns home, he is ambushed by the man who shot him. Holding Jef at gunpoint, the man offers a fresh start. He pays Jef for the hit on Martey, as well as for an upcoming one. Jef overpowers the man and forces him to disclose the identity of his boss: Olivier Rey.
Dozens of undercover police attempt to tail Jef in the Paris Métro, but he evades them. He visits Jane and assures her everything will work out, then drives to Rey's house, which is the same home Valérie took him to earlier in the film, though she is not there. Jef kills Rey and returns to the nightclub, this time making no attempt to avoid being seen. He checks his hat, but leaves his hat-check ticket on the counter, and puts on white gloves, which he wears when carrying out his hits, in full view of everyone. He approaches the stage where Valérie is performing. She quietly advises him to leave, but he points his gun at her. Strangely unafraid, she asks him why he is doing this, and he responds he was paid to do so. Suddenly, Jef is shot four times by policemen who had been waiting for him. When the Commissaire inspects Jef's gun, however, he discovers it was not loaded.
